Tunable lasers have wide applications in optical communication field, especially in recent years when the optical network is developing towards a dynamic optical network. The future optical platform will need to support a variety of different data rates and types of customer data service. Based on a fixed frequency interval (e.g., 50 GHz, 100 GHz), existing DWDM system can not make efficient use of spectrum resources of optical fibers, thereby limits the effective expansion of optical fiber transmission capacity.
To solve this problem, a concept of flexible wavelength grid was proposed in this industry, i.e., the wavelength interval uses a variable wavelength grid which is determined by frequency bins with a certain range of frequencies. The center frequency of the wavelength grid is defined as the center of frequency bin whose width is an integral multiple of a standard frequency bin size, and the frequency bins can be combined in an arbitrary way. Therefore, ITU-T (ITU Telecommunication Standardization Sector) expanded the grid standard, and defined the grid size as 12.5 GHz, frequency interval as an integer multiple of 12.5 GHz, and the center frequency of the frequency bin as 193.1±n×0.00625 THz (n is an integer). Flexible wavelength grid can allocate a suitable width of frequency bin, according to rate and modulation pattern requirements of the transmission signal, to realize a transmission system with high spectral efficiency and large capacity, such as the Orthogonal Frequency Division Multiplexing System (OFDM). As a result, the role of tunable laser with flexible wavelength grid tuning function and narrow linewidth is becoming increasingly prominent.
Currently, the main two methods for realizing flexible wavelength grid are as follows: one method realized the continuous wavelength adjustment within a certain range to cover any grid; and the other realized the adjustment with grid size as small as possible.
The US patent (U.S. Pat. No. 6,847,661 B2) published a continuously tunable external-cavity laser based on Micro-Electron-Mechanical System (MEMS). In order to realize a larger continuous adjustment range, drivers arrayed in a fan shape make the external-cavity MEMS reflector rotate around an imaginary axis at a distal end. Thus, structure of the MEMS driver is very complex, poor in reliability, especially in shock resistance.
US patent (U.S. Pat. No. 7,991,024 B2) published a kind of liquid crystal external cavity tunable laser, using a liquid crystal filter as the channel selection element and a Fabry-Perot (F-P) etalon to generate a fixed frequency grid. The Fabry-Perot etalon is to limit the output wavelength at the standard ITU-T wavelength and suppress longitudinal modes adjacent to the lasing wavelength. In this way, its Free Spectral Range (FSR) can be set at a standard ITU-T interval, such as 50 GHz or 100 GHz. The liquid crystal filter is actually a liquid crystal Fabry-Perot interferometer, which has only one transmission peak within the operating band to select that of one etalon in this band. However, since single LC filter has a wide bandwidth, normally about 1 nm, when the grid interval of Fabry-Perot etalon is fixed to be small (e.g. 12.5 GHz), application of the scheme is rather limited.